1. Field of the Invention
The present invention relates to novel piezoelectric resonators and electronic components containing the novel piezoelectric resonators, and more particularly, to a piezoelectric resonator which maximizes the effective use of mechanical resonance of the novel piezoelectric member, and electronic components containing the piezoelectric resonator, such as an oscillator, a discriminator, and a filter.
2. Description of the Related Art
FIG. 34 is a perspective view of a conventional piezoelectric resonator. A piezoelectric resonator 1 includes a single piezoelectric substrate 2 having, for example, a rectangular plate shape as viewed from above. The single piezoelectric substrate 2 is polarized in a thickness direction of the substrate 2. On two opposite surfaces of the single piezoelectric substrate 2, electrodes 3 are disposed. When a signal is input between the electrodes 3, an electric field is applied to the single piezoelectric substrate 2 in the thickness direction and the single piezoelectric substrate 2 vibrates in the longitudinal direction.
In FIG. 35, there is shown a piezoelectric resonator 1 in which electrodes 3 are disposed on two surfaces of a single piezoelectric substrate 2 having a square plate shape as viewed from above. The single piezoelectric substrate 2 of the piezoelectric resonator 1 is polarized in the thickness direction. When a signal is input between the electrodes 3 in the piezoelectric resonator 1, an electric field is applied to the single piezoelectric substrate 2 in the thickness direction and the single piezoelectric substrate 2 vibrates in a square-type vibration mode (in the plane direction).
These piezoelectric resonators are unstiffened type resonators, in which the vibration direction differs from the direction of polarization and the electric field. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator. In stiffened piezoelectric resonators, the vibration direction, the direction of polarization, and the direction in which an electric field is applied are the same. An unstiffened piezoelectric resonator has a relatively small frequency difference .DELTA.F between the resonant frequency and the antiresonant frequency. This leads to a drawback in which a frequency-band width is too narrow when an unstiffened resonator is used as an oscillator or a filter. Therefore, the degree of freedom and flexibility in resonator characteristics design is limited in such an unstiffened piezoelectric resonator and electronic components using such piezoelectric resonators.
The piezoelectric resonator shown in FIG. 34 uses the first-order resonance in the longitudinal mode. It also generates, because of its structure, large spurious resonances in odd-number-order harmonic modes, such as the third-order and fifth-order modes, and in the width mode. To suppress these spurious resonances, some solutions have been considered, such as polishing, increasing mass, and changing the shape of the electrodes. These solutions increase manufacturing cost.
In addition, since the piezoelectric substrate has a rectangular plate shape, the thickness of the substrate cannot be reduced because of restrictions in strength. Therefore, the distance between the electrodes cannot be reduced and a capacitance between terminals cannot be increased. This makes it extremely difficult to achieve impedance matching with an external circuit. To form a ladder filter by alternately connecting a plurality of piezoelectric resonators in series and in parallel, the capacitance ratio of the series resonator to the parallel resonator needs to be made large in order to increase attenuation. Because a piezoelectric resonator has the structure and shape restrictions described above, however, large attenuation cannot be obtained.
The piezoelectric resonator shown in FIG. 35 uses square-type first-order resonance (in the plane direction). As a result of its structure, large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are also generated. Since the piezoelectric resonator requires a large size as compared with a piezoelectric resonator using the longitudinal vibration in order to obtain the same resonant frequency, it is difficult to reduce the size of piezoelectric resonator. When a ladder filter is formed by a plurality of piezoelectric resonators, in order to increase the capacitance ratio between the series resonator and the parallel resonator, the resonators connected in series have a substantially increased thickness and electrodes are formed only on part of a piezoelectric substrate which reduces the capacitance. In this case, since the electrodes are formed only partially, the difference .DELTA.F between the resonant frequency and the antiresonant frequency as well as the capacitance is reduced. The resonators connected in parallel are accordingly required to have small .DELTA.F. As a result, the piezoelectricity of the piezoelectric substrate is not effectively used, and the transmission band width of the filter cannot be increased.
The inventors desired to provide a piezoelectric resonator having small spurious resonance and a large difference .DELTA.F between the resonant frequency and the antiresonant frequency. In such a piezoelectric resonator, a plurality of piezoelectric layers and a plurality of electrodes are alternately laminated to form a base member, and the plurality of piezoelectric layers are polarized in the longitudinal direction of the base member. This laminated piezoelectric resonator is a stiffened resonator type, and includes piezoelectric layers in which the vibration direction, the direction of polarization, and the direction in which an electric field is applied are the same. Therefore, as compared with an unstiffened piezoelectric resonator, in which the vibration direction differs from the direction of polarization and electric field, the stiffened piezoelectric resonator has a larger electromechanical coupling coefficient and a larger frequency difference .DELTA.F between the resonant frequency and the antiresonant frequency. In addition, vibrations in modes such as the width and thickness modes, which are different from the basic vibration, are unlikely to occur in a stiffened piezoelectric resonator.
Since in the piezoelectric resonator having this lamination-structure, each piezoelectric layer constituting the base member has the same length in the longitudinal direction of the base member as the other piezoelectric layers and each electrode has the same area as the other electrodes, the capacitance between each pair of adjacent electrodes is the same and the driving force piezoelectrically generated by each piezoelectric layer is also the same.
In the longitudinal basic vibration, a stronger driving force is required at a portion of the base member located closer to the center thereof because of the large mass of this portion as compared to an end portion of the base member. Therefore, the piezoelectric resonator has an insufficiently large electromechanical coupling coefficient and thus .DELTA.F is not sufficiently large.
In such piezoelectric resonator, high-order-mode vibration is unlikely to occur. Since the capacitance between each pair of adjacent electrodes is constant because each piezoelectric layer has the same length in the longitudinal direction of the base member and each electrode has the same surface area, however, charges generated in each piezoelectric layer by odd-number-high-order-mode vibration, such as the third-order and fifth-order vibrations, are not sufficiently canceled, causing high-order-mode spurious vibrations.